Injection molding is a well-known process which may be used for the fabrication of complexly shaped plastic or metal objects or parts. In the injection molding process, a molten thermoplastic or metal material, such as aluminum, is introduced into a mold and allowed to set or cure by cooling. Once the plastic or metal is set or cured, the mold is opened, and the molded object is released. The temperature of the injection mold is preferably controlled so that the mold is at the proper temperature when the molten material is injected into the mold and such that the object formed in the mold is set or cured at an optimal rate to both maintain the quality of the molded object while minimizing the setting or curing time to maximize production rates.
Generally, an injection mold is cooler than the molten material which is injected into it. Thus, as hot molten material is injected into a mold, the mold absorbs heat from the molten material. This heat must be removed from the mold, and, therefore, from the molten material, before the molten material will set or cure. As a series of objects are molded successively in a mold, the temperature of the mold will tend to increase toward the temperature of the molten material being injected into the mold.
It is generally desirable to maintain the temperature of an injection mold at a level which allows a plastic or metal object being molded to exhibit the least possible amount of shrinkage and distortion during the setting or curing process. It is also important to maintain the mold at a consistent operating temperature so as to ensure uniformity among replications of the object being molded. It is also desirable to remove excess heat from the mold promptly after the molding of an object so as to more quickly prepare the mold for the subsequent introduction of a molten material. Increased production efficiency can be achieved if the mold temperature is quickly brought down to the desired operating temperature between individual object molding cycles.
Temperature control of an injection mold is typically accomplished by circulating cooling fluid through channels fashioned in the walls of the mold. Conventionally, the cooling fluid is heated to a desired ideal operating temperature for cooling the mold and then is circulated through the mold before the first injection or "shot"of hot plastic or metal material is introduced into the mold. The temperature of the mold initially increases upon the introduction of the hot molten material, but is restored to the desired operating temperature by the continuous circulation of the cooling fluid, the temperature of which is maintained at the ideal operating temperature. In order to maintain the desired mold operating temperature, the cooling fluid may be circulated through the mold substantially all the time that the mold is being used to make successive replications of the object being molded.
Methods and devices for controlling the temperature of a fluid-cooled injection mold without the need for a continuous flow of cooling fluid are described in U.S. Pat. Nos. 4,354,812 and 4,420,446 to Horst K. Wieder, et al. These patents describe methods by which an injection mold can be maintained at a desired operating temperature using a cooling fluid which need not be elevated to or maintained at an ideal operating temperature. Accurate control of the temperature of an injection mold can be achieved by mounting a temperature sensor onto or within the mold. The temperature sensor provides an output signal indicative of the mold temperature. If the sensed mold temperature exceeds a selected control temperature level, a valve is opened to allow cooling fluid to enter the cooling channels in the mold, to thereby cool the mold. When the temperature sensor indicates that the mold is cooled below the control temperature, the valve is closed. Since cooling fluid is not continuously pumped through the mold cooling channels, the cooling fluid need not be heated to a particular operating temperature and the consumption of cooling fluid is reduced.
Another method of injection mold temperature control is described in U.S. Pat. No. 5,427,720 to Kotzab. Typically, a plurality of cooling channels are formed in an injection mold to provide cooling fluid to the mold. This patent describes determining, empirically or by calculation, a selected distribution profile for distributing cooling fluid among the cooling channels to achieve the desired amount of cooling of the injection mold. Depending upon the shape of the object being molded, certain portions of the injection mold may require more cooling than others. At the same time during each molding cycle, a temperature sensor signal is used to determine the temperature deviation of the mold from a desired temperature. Simultaneously, valves are opened to provide pulses of cooling fluid through the cooling channels in the pre-determined distribution profile. The duration of the cooling pulses is determined by the measured temperature deviation.
For some applications, the "pulse" cooling injection mold temperature control schemes just described may employ ordinary tap water as the cooling fluid. However, for many molding operations, the operating temperature of an injection mold is very high. For example, the operating temperature of an injection mold for a high temperature molding process may be 300.degree. F. or higher, and the molten material injected into the mold may typically be at 700.degree. F. or higher. It has been believed that ordinary tap water cannot be used as the cooling fluid for such high temperature molding operations. It has been thought that water would instantly turn to steam upon entering the cooling fluid channels of the high temperature injection mold, and thus could not effectively reduce the mold temperature.
Petroleum-based oils or mineral heat transfer fluids are typically employed as cooling fluids for controlling the mold temperature of high temperature injection molding operations. The use of such materials for high temperature injection mold cooling has several important limitations, however. Such fluids have an inherently poor heat transfer rate. Thus, the time needed during a production cycle to bring the injection mold to the desired operating temperature using such fluids is relatively long, thereby increasing the cycle duration, and decreasing the production rate. Furthermore, petroleum based oils are difficult to work with and potentially dangerous. The combination of petroleum-based oil and high temperatures presents a fire hazard. The use of oil-based cooling fluids can also adversely affect the quality of a molded object. Hydrocarbon molecules from the cooling oil can get into the mold itself. These molecules will leave contamination deposits on the molded plastic or metal object. These contamination deposits can adversely affect the quality and appearance of the molded object. In particular, contamination deposits on an aluminum injection molded or die cast object, caused by oil based cooling fluid contamination, will prevent finishing of the aluminum object, such as high surface gloss finishing or metal-like chrome metalization, in the affected area. If the aluminum object cannot be finished properly, it must typically be scrapped.